In recent years, development of vehicles that allow fuel consumption to be reduced in consideration of the environment is advanced. Pneumatic tires are mounted to the vehicles. The pneumatic tires act to support vehicle bodies, and to transmit power with respect to road surfaces. The tires exert influence on fuel efficiency of the vehicles.
Development of tires is also advanced in view of achieving reduction in fuel consumption of vehicles. In this development, examination is made so as to focus on rolling resistance of the tires. An exemplary examination is disclosed in JP2005-219537.
Conventionally, for commercially available replacement tires, multiple sizes of the tires are lined up for one tread pattern. Up to more than 70 sizes of tires are lined up for one tread pattern in some cases. The tire sizes are defined in the standard (for example, JATMA standard). The tire size is represented as, for example, 195/65R15. 195 represents a nominal width (mm), 65 represents a nominal aspect ratio, R represents radial, and 15 represents a nominal rim diameter (inch). Among them, in a manner for extending and developing tire widths for one aspect ratio, a representative tire width for the one aspect ratio is firstly selected. A profile of a mold for the representative tire width is designed. A profile of a mold for each tire width is determined so as to have a shape similar to the representative profile. Subsequently, in consideration of a required rubber gauge, a carcass profile for each tire width is designed. The carcass profile refers to a shape represented by the center line (carcass line) of the thickness of the carcass.
As described above, in order to extend and develop the tire widths, the carcass profile needs to be designed for each tire width. The number of steps in the designing is increased according to the number of the tire widths to increase a load. Since the carcass profile is designed for each tire width, variation in designing may occur. As a result, the carcass profile for a tire having a certain tire width greatly changes between before inflation and after inflation, so that excess distortion may be generated in rubber and cords. In other words, the carcass profile may deviate from a natural equilibrium shape.
The carcass profile based on the natural equilibrium shape theory represents an equilibrium shape of the carcass which is formed such that, when the tire is inflated to a normal internal pressure, a tensile force of the carcass balances with a tire internal pressure and a reaction force from a belt in a case where forces other than the tire internal pressure and the reaction force from the belt are not applied. The carcass based on the natural equilibrium shape theory is varied so as to approximate the entirety of the shape thereof to almost similar shapes according to increase or reduction of the internal pressure. This means that the carcass is almost uniformly deformed according to increase or reduction of the internal pressure, and an amount of the deformation is small. In a case where the carcass is not in the equilibrium state in which the carcass profile changes so as to have similar shapes according to change of the internal pressure, that is, the carcass does not have the natural equilibrium shape, durability of the tire may be reduced due to concentration of stress or distortion.
In JP08-142601, a method for manufacturing a radial tire by applying the natural equilibrium shape theory is suggested. In this manufacturing method, protrusion of the carcass profile according to change of tire internal pressure is defined in order to improve vibration resistance and ride comfort. Also in JP08-142602, a method for manufacturing a radial tire by applying the natural equilibrium shape theory is suggested. In this manufacturing method, a radius (radius of curvature) of the carcass profile is defined in order to improve vibration resistance and ride comfort.
However, also in a case where the carcass profile is designed by applying the natural equilibrium shape theory, the carcass profile is designed for each tire width, whereby the number of steps in the designing is increased according to the number of the tire widths to increase a load.